Underground » Strata Control and Windblasts
Structurally related falls on the southern longwalls at North Goonyella Coal Mine are a continuing problem. Geotechnical monitoring, primary/secondary support design systems, in combination with standard Face Management Systems have helped reduce the number of fall events. NGCM's current efforts at maintaining a stable geotechnical working environment has not been able to completely prevent falls from occurring.
North Goonyella Coal Mines (NGCM) personnel have identified that longwall equipment mechanical issues may have contributed to the difficulty mining through faulted zones. Consequently preconsolidation of the strata in advance of mining may provide sufficient binding to enable a more continuous fault driveage procedure.
Historically, following falls along the extraction face, the majority of consolidation work has been remedial polyurethane resin injection which to date has proved very expensive in terms of cost of resin and down time on the Longwall.
The use of preconsolidation techniques to stabilise and strengthen geologically disturbed and hydrologically affected strata is an already established field of expertise in civil engineering. The coal mining industry however, has limited experience in this field.
NGCM has recognised that the use of preconsolidation could substantially reduce the frequency of falls and consequently return more consistent coal productivity. Following on from this, NGCM submitted a research grant proposal to ACARP in May 1997 for the investigation, planning, implementation, monitoring and consequent documentation of the process used to formulate a site specific preconsolidation technique.
Work carried out to date has involved the accumulation of relevant information on preconsolidation techniques and based on this data three preconsolidation trials have been implemented in the Main Headings and Longwall 5 South. The technique for the first two trials involved the injection of a sulphate resisting ground Portland Micro fine cement into a number of directional in-seam boreholes. The injection of the micro fine cement in these trials was carried out from the collar of the bore holes via a grouted in 25mm standpipe. The third trial was set up purely to prove a working technique for injection of micro fine cement and varied to the previous two trials in that injection was achieved by selective down the hole pumping. This was achieved via interconnected hydraulic tubing and an insitu wire line packer arrangement.
The third trial proved quite a successful technique for micro fine cement application as defined by the volume of cement injected and would be the basis for a larger scale exercise.
Due to a possible change in company strategy with regards to Longwall 5 South extraction it may not be possible to measure the effect of the third trial, ie influence of permeation and binding potential.
NGCM is confident that is has systematically determined a technique for micro fine cement injection.
This report details the process used to define the site specific preconsolidation technique and all relevant field data and observations.